The present invention relates to a melt spinning apparatus. More particularly, it is concerned with a melt spinning apparatus suitable for use in multifilament spinning for pitch-based carbon fibers.
In conventional melt spinning of high polymers, great efforts are made to prevent foreign matter from being deposited and growing on the vicinity of a spinneret nozzle which would be a main cause of problems in the spinning process, and also to prevent the breakage and unevenness in diameter of spun yarn. For example, in a melt spinning apparatus for such high polymers as polypropylene, nylon and polyester, usually a spinning pack, a delayed cooling portion and a forced cooling portion having a cooling air blowing face are arranged successively from above, and as a drawing zone for spun yarn, a quenching column is used over an area of at least 30 cm, usually 50 to 100 cm, to provide for a uniform cooling air temperature, humidity and wind velocity, and consideration is given to remove volatile matter or fumes contained in high polymers.
On the other hand, petroleum or coal pitch, as compared with the above-mentioned high polymers, is small in average molecular weight, ranging from about 600 to 2,000, and the molecular weight distribution thereof is not always narrow. From the standpoint of improving its spinnability and the performance of carbon fiber obtained by carbonizing pitch fiber, an attempt has been made to adjust the molecular weight distribution by subjecting pitch to a solvent fractionation and thereby removing low and high boiling components. During melt spinning, however, it is unavoidable for a trace amount of a low boiling component to become fume and stain in the vicinity of the spinneret and it is difficult to keep a stable spinning for a long time. Besides, because of a small average molecular weight, the dependence of melting viscosity upon temperature is extremely large and even a slight change in temperature causes a large change in viscosity. Further, the pitch spinning temperature is high, generally not lower than 300.degree. C., and the viscosity is extremely low, ranging from 1,000 to 500,000 cP. Therefore, if a high tension is used with a view to obtaining a fine yarn under insufficient cooling, there will occur breakage of yarn, while if spun yarn is cooled excessively, it will be impossible to obtain a fine yarn because pitch fiber solidifies rapidly before it is drawn. Additionally, if cooling is not uniform, there will occur unevenness in yarn diameter.